Assay devices in the form of biochips are well known in the field of immunoassays. Such biochips are often placed in storage wells and are deposited with an array of localised reactive sites containing potentially many different reactive species, for example, different proteins. These reactive species react with a respective different analyte in a sample supplied to the chip. Following removal of the unbonded sample, the biochip can then be examined to determine the presence or absence of the respective analytes.
An example of an assay assembly comprising a storage well and biochip is described in more detail in EP 0988893 A1. This application is incorporated herein by reference. The use of a storage well protects the assay device (in this case, the biochip) while it is being handled during an immunoassay process, and helps to prevent reactions sites from being damaged.
A problem with the assay assemblies described in the prior art is that the storage well should be sealed in order to reduce vapour loss during the heating stage of the immunoassay. When the well is heated during the run of an assay, depending on the temperature, there is the risk of fluid loss via vapour through the opening of the well cavity and the subsequent contamination of the environment, lab equipment or a biochip analyser. The assay could also be affected as the fluid loss through vapour would change the volumes of fluid in the biochip well. If the vapour loss were to be corrected for by adding additional fluid, this would incur an additional cost, and there would still be risk of contamination of the environment. Traditionally, the reduction of vapour loss is achieved by one of three means.
Firstly, oil is often used to form a seal on top of the fluid during heating. However, the scientist would be constrained to use only fluids of certain densities in assays, and would have to follow material disposal guidelines for oils. The oil would also coat the surface of the biochip if the fluid were removed from the well as part of the assay, or reduce sensitivity of detection by forming a layer between the camera and the biochip.
Alternatively, an adhesion film or foil is used to seal the assay during heating. This is suitable for manual assays, but the foil needs to be completely removed for imaging. Further, applying the foil as part of an automated process is a complicated, costly and space consuming task for a biochip analyser to perform and so is undesirable.
Finally, it is known in the art to use caps which are screw or plug based to seal the storage well during the run of an assay. However, such caps require complicated robotics to form a seal. Further, the thread in a screw cap is not vapour proof, resulting in some evaporation and loss of fluid through the thread at high temperatures or during prolonged time periods. Additionally, plug caps have an inherent risk of aggressive depression at high liquid temperatures, causing a sudden jerking motion which can result in droplets of fluid escaping. Therefore, both screw caps and plug caps harbour risks of contamination where hazardous fluids are present.